Hi flossy1971;
Normally the Engineer's code should be entered to go into 'programming mode'.
Depending on who installed your alarm system? They may have changed the Engineer Code? But the factory default is '7890'.
The procedure is as follows:
1. Press '0' followed by 'Enter' (or '#').
2. Enter Engineer's code (default: 7890).
If you've entered the correct code, all LED's except for power, fault & service should be flashing. If this is the case, then you can go onto next step & safely open the unit without any problem to replace the battery.
3. Replace the battery & close the unit back up, making sure no loose wires will be trapped by lid.
4. Press '99' followed by 'Enter' (or '#'). This returns the unit back to it's normal state.
Hope this helps.
Simon
September 2008